1. Field of the Invention
This invention pertains to the formation of low voltage aluminum electrolytic capacitor foil and in particular to the hydration of said foil before drying to increase the capacitance of the foil and the efficiency of formation of the foil without losing capacitance due to uncontrolled reaction.
2. Description of the Prior Art
Since the introduction of aluminum electrolytic capacitors around the turn of the century, there has been a continuing struggle to increase the capacitance per unit volume of the devices. Given the standard formulae for the capacitance of a parallel plate capacitor, increased capacitance is obtained either by increasing the true to apparent area ratio through etching or to increase the ratio of dielectric constant to dielectric strength through formation. The present invention is concerned with formation.
The first commercial capacitors used boric acid formation solutions to produce the aluminum oxide dielectric. Because of the stability problems and the solution costs, phosphate solutions displaced boric acid for low voltage use. In time, it was discovered that pretreatment such as hydration, heat treatment and annealing would produce improved formation efficiency and hence speed of formation and higher capacitance. These foils were inherently unstable. Hence, the hot water, heat treatment, and various other depolarization procedures were developed to permit the use of these gain improving, energy reducing processes.
The difficulty remained with low voltage foil because of its very fine etched tunnels, approximately a few tenths of a micron in diameter. Hydration is produced by a violet reaction with hot water which is difficult to control externally to the thicknesses required. Another approach involving high temperature heat treatment, produced high gain and high efficiency formation. However, the nature of this oxide was such as to be very difficult to produce stable, low-leakage-current formed foil. Experience showed that the proper annealing of the foil after etching had some beneficial effects on capacitance without the associated difficulties of heat treated foil. Currently, development work has focused on the use of dicarboxylic acids as formation ionogens. It has been found that these electrolytes produce capacitance equivalent to or greater than heat treatment at voltages above 25 EFV. This process produced high stability, high gain foil.
The present application is based on an understanding of the mechanism of gain enhancement offered by dicarboxylic acid formation. U.S. Pat. No. 4,252,575 teaches that attempts have been made to hydrate low voltage foil by use of passivators to control hydration. It is current in the art to use 95.degree. to 100.degree. C. deionized water with passivators to slow the reaction. This method is very difficult to control and has disadvantageous passivators which limit the capacitance increase possible in certain formation solutions.
Deductive reasoning shows that all known high gain formation processes involve treatments between etch and formation which deposit and/or modify the surface layer, a thermal oxide or hydrate. The dicarboxylic acid process mechanism is not known but could conceivably be aided by the presence of such a layer. Thermal layers are difficult from a control and stability point of view. The difficulty with hydration is that long induction times render control difficult. The induction time appears to be related to the fact that hydration cannot begin until the existing thermal oxide left by annealing and/or drying has been dissolved or penetrated. With this understanding, the control measure suggested by the present invention is to eliminate the induction time by depositing the hydrous surface layer before drying and annealing. This can be accomplished after final rinse in the etch procedure by exposing the foil to heated deionized water at a temperature low enough to control the reaction rate.